The invention relates to a method for a reliable interface between a telephone having a card with subscriber identity and the network when dialing specific preprogrammed telephone numbers in a telephone system. In this way, the user of a telephone is to be allowed only to dial specific preprogrammed telephone numbers.
In the GSM (Group Special Mobile) mobile radio system, this feature is designated by FDN (Fixed Dialing Number). The permitted numbers and the fact of whether the restriction is switched on or off is stored on a smart card. If the feature "restriction" is switched on, the telephone carries out a check when a number is entered to see whether the latter is enabled, and carries out a call setup in the event of a positive result. However, there is the problem here that the interface between the card and the telephone is freely accessible, in other words the presence of any desired number in the list of permitted numbers can very easily be simulated by intercepting the relevant commands, for example.